1. Field of the Invention
The present invention relates to a pressure-sensitive resistor and a pressure-sensitive sensor having the pressure-sensitive resistor.
2. Description of the Related Art
As a pressure-sensitive sensor has been known a sensor using a volume resistance variation in a resistor when pressure is applied to the resistor (“SENSOR TECHNIQUE”, Vol. 19, No. 9, 1989). In this pressure-sensitive sensor, a great deal of pressure must be applied to the sensor to have a large resistance variation rate and thus it is generally unsuitable to detect a low pressure.
JP-A-2003-106912 proposes a pressure-sensitive sensor using a contact resistance variation on the surfaces between electrical contact points. This pressure-sensitive sensor has a pair of electrodes and two layers of pressure-sensitive resistance materials which are formed on the respective electrodes through a predetermined gap. The pair of electrodes and the two layers of the pressure-sensitive resistance materials are provided between a pair of base films. The surface of each electrical conductive particle constituting the pressure-sensitive resistant material is coated by extremely thin polymer. When a pressure is applied to the base films, the contact area between the pressure-sensitive resistant materials is varied in accordance with the applied pressure, and this variation of the contact area causes a variation in true contact area resistance (concentrated resistance) between the electrodes. The true contact area resistance is based on the contact area, and the resistance variation is little observed when the contact area is saturated.
When the pressure-sensitive resistant materials are deformed by the applied pressure in a state where the pressure-sensitive resistant materials contact with each other, the distance between the polymer-coated electrical conductive particles at the contact position between the pressure-sensitive resistance materials is varied. Therefore, the tunnel conduction between the electrical conductive particles is varied and it appears as coating film resistance variation. This pressure-sensitive sensor achieves linear resistance variation in a broad pressure range by using both the resistance variations described above.
However, when pressure of 1 to 20 kPa for a detection of a passenger in a vehicle or for a measurement of a body pressure distribution of a human body is mainly set as a detection range, there may occur such a case that even when an applied pressure is increased, the contact area between the pressure-sensitive resistant materials is not increased because the applied voltage is low. In this case, no true-contact area resistance variation occurs, and thus no linear resistance variation can be obtained in the above pressure range.